hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...
[[Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems]]

Can you attach a screenshot?
*http://en.wikipedia.org/wiki/Screenshot
*https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem
Use a compressed image type like PNG or JPG to save the screenshot.